Three-tube cathode ray tube (CRT) light sources have hitherto been mainly used as light sources for projection televisions. A demand for digitalization, enhanced definition, and compactification in recent years has led to expanded use of single-tube light sources such as LCDs and DMDs. The single-tube light sources such as LCDs and DMDs have advantages over the above CRTs. At the present time, however, the output of the light source does not reach that of CRTs yet.
On the other hand, the projection television should have given color reproduction as displays, and the color temperature is generally set to about 10,000 K. In order to maintain the above color temperature, it is common practice to regulate the light source and the circuit.